It has been suggested that the circadian clock system in Arabidopsis thaliana is associated with increased biomass, determination of the time of flowering, and environmental stress responses (Non-Patent Document 1). As a result of research conducted using Arabidopsis thaliana, three plant clock-associated genes (i.e., the circadian clock-associated 1 (CCA1) gene, the late elongated hypocotyl (LHY) gene, and the timing of cab expression 1 (TOC1) gene) have been discovered, and the mechanisms of plant circadian rhythm have been proved to be based on the transcriptional feedback loop of such a plurality of genes. Among these, the TOC1 gene is one of pseudo-response regulator genes. At present, a total of 5 genes (i.e., PRR3, PRR5, PRR7, and PRR9, in addition to TOC1 (PRR1)) have been identified as pseudo-response regulator genes in Arabidopsis thaliana, and the expression levels thereof increase and decrease in the order of PRR9, PRR7, PRR5, PRR3, and PRR1 (TOC1) from dawn toward evening in a phase-graded manner.
The present inventors had focused on and studied the prr9 prr7 prr5 triple mutant of Arabidopsis thaliana (i.e., the d975 mutant). As a result of previous studies, the d975 mutant was found to exhibit industrially useful phenotypes such as flowering stem elongation (Non-Patent Document 3), delayed flowering (Non-Patent Document 2), and higher resistance against low temperature or dehydration (Non-Patent Document 4), in addition to a breakdown of the circadian clock system (Non-Patent Document 2). However, since the three PRR genes complement their functions each other, a single mutant would not exhibit a remarkable phenotype (Non-Patent Document 2). In addition, conferring of a useful phenotype by deletion of the three PRR genes as targets would be attended with the difficulty in preparation of a multiple mutant. Further, since the biochemical functions of PRR9, PRR7, and PRR5 proteins are unknown, suppression of the functions of the PRR proteins with the use of a low-molecular-weight compound or the like is not practical technically.
An example of a technique for conferring low temperature resistance or dehydration resistance is a method in which expression of the DREB1A gene or similar genes is induced under the control of a dehydration stress responsive promoter of Arabidopsis thaliana (Patent Document 1 and Non-Patent Document 5). While this method may be sufficient to confer stress resistance, increased biomass is not mentioned.
Increased biomass is observed for a mutant of the clock-associated gene, GIGANTEA (gi) (Non-Patent Document 6); however, this mutant is more susceptible to low temperature stress (Non-Patent Document 7). Thus, a plant exhibiting increased biomass and environmental stress resistance cannot be prepared by a method of deleting the function of this gene as a target.
Slightly decreased expression levels of the clock-associated genes (i.e., the circadian clock associated 1 (CCA1) gene and the late elongated hypocotyl (LHY)) are said to be correlated with the increased biomass (Non-Patent Document 8); however, a line in which expression of the CCA1 gene and the LHY gene was completely suppressed (i.e., the ccal lhy double mutant) rather exhibits a decreased biomass (Non-Patent Document 9). Thus, it is considered difficult to increase the biomass by targeting these genes.
While the biological clocks of long-day plants are almost the same as those of short-day plants (Non-Patent Document 10), seasonal responses utilizing such biological clocks are known to differ from each other (Non-Patent Document 11). Accordingly, the degree of effectiveness of conferring a useful phenotype on a short-day plant by modifying a clock gene of a long-day plant (i.e., Arabidopsis thaliana) is unknown.    (Patent Document 1) JP Patent Publication (Kokai) No. 2000-116260 A    (Non-Patent Document 1) Harmer, S. L., Hogenesch, J. B., Straume, M., Chang, H. S., Han, B., Zhu, T., Wang, X., Kreps, J. A., and Kay, S. A., 2000, Orchestrated transcription of key pathways in Arabidopsis by the circadian clock, Science 290 (5499): 2110-2113    (Non-Patent Document 2) Nakamichi, N., Kita, M., Ito, S., Yamashino, T., and Mizuno, T., 2005, Pseudo-response regulators, PRR9, PRR7 and PRR5, together play essential roles similar to the circadian clock of Arabidopsis thaliana, Plant Cell Physiol., 46 (5): 686-698    (Non-Patent Document 3) Yamashino, T., Ito, S., Niwa, Y., Kunihiro, A., Nakamichi, N., and Mizuno, T., 2008, Involvement of Arabidopsis clock-associated pseudo-response regulators in diurnal oscillations of gene expression in the presence of environmental time cues, Plant Cell Physiol., 49 (12): 1839-1850    (Non-Patent Document 4) Nakamichi, N., Kusano, M., Fukushima, A., Kita, M., Ito, S., Yamashino, T., Saito, K., Sakakibara, H., and Mizuno, T., 2009, Transcript profiling of an Arabidopsis pseudo-response regulator arrhythmic triple mutant reveals a role for the circadian clock in cold stress response, Plant Cell Physiol., 50 (3): 447-462    (Non-Patent Document 5) Kasuga, M., Liu, Q., Miura, S., Yamaguchi-Shinozaki, K., and Shinozaki, K., 1999, Improving plant drought, salt, and freezing resistance by gene transfer of a single stress-inducible transcription factor, Nat. Biotechnol., 17 (3): 287-291    (Non-Patent Document 6) Redei, G., 1960, Supervital mutants of Arabidopsis, Genetics 47: 443-460    (Non-Patent Document 7) Cao, S., Ye, M., and Jiang, S., 2005, Involvement of GIGANTEA gene in the regulation of cold stress response in Arabidopsis, Plant Cell Rep., 24 (11): 683-690    (Non-Patent Document 8) Ni, Z., Kim, E. D., Ha, M., Lackey, E., Liu, J., Zhang, Y., Sun, Q., and Chen, Z. J., 2009, Altered circadian rhythms regulate growth vigour in hybrids and allopolyploids, Nature 457 (7227): 327-331    (Non-Patent Document 9) Mizoguchi, T., Wheatley, K., Hanzawa, Y., Wright, L., Mizoguchi, M., Song, H. R., Carre, I. A., and Coupland, G., 2002, LHY and CCA1 are partially redundant genes required to maintain circadian rhythms in Arabidopsis, Dev Cell 2 (5): 629-641    (Non-Patent Document 10) Murakami, M., Tago, Y., Yamashino, T., and Mizuno, T., 2007, Comparative overviews of clock-associated genes of Arabidopsis thaliana and Oryza sativa, Plant Cell Physiol., 48 (1): 110-121 (Non-Patent Document 11) Hayama, R., Yokoi, S., Tamaki, S., Yano, M., and Shimamoto, K., 2003, Adaptation of photoperiodic control pathways produces short-day flowering in rice, Nature, 422 (6933): 719-722